The present invention relates to an optical scanner for use in a laser beam printer and other apparatus.
An optical scanner conventionally used in a laser beam printer and other apparatus comprises a light source such as a semiconductor laser, a collimator lens that collimates the light beam issued from the light source, a rotating polygonal mirror that deflects the collimated beam for scanning, and an imaging lens that focuses the deflected light to form a beam spot on a scan surface.
The imaging lens is required to have the following two aberrational characteristics: i) a specified negative value of distortion that is necessary to achieve scanning at uniform velocity, and ii) a smaller curvature of the field that reduces the beam spot size to nearly the diffraction limit, thereby producing a flat image plane.
The imaging lens can be composed of one or more lens elements. If good aberrational characteristics are required, many lens elements are used; if cost is important, a single lens is used. To provide further improved aberrational characteristics, the use of an imaging lens having an aspheric surface is often used today, as typically taught in JPA 92/50908.
A spherical lens surface has the same curvature in every position. On the other hand, an aspheric lens surface has varying local curvatures in different positions. Therefore, the aspheric imaging lens described in JPA 92/50908 has a problem in that if the light beam passing through the lens has a diameter greater than a certain value, the local curvature varies with position even within a cross section of the beam on the aspheric surface. As a result, the wavefront of the beam that has undergone transformation by the aspheric surface is disturbed to become deteriorated in imaging characteristics, whereby the shape of the beam spot is deformed. The amount of asphericity can range from one extreme that has a negligible departure from a spherical surface to another that has a point of inflection, changing from concave to convex or conversely in the center of the optical axis and at either edge. The deformation of the beam spot is particularly noticeable in the latter case.
With the recent advances in laser beam printer technology for higher resolution, there is a growing need for optical scanners to form even smaller beam spots. A Gaussian beam is such that in order to produce a beam spot smaller with a lens having the certain focal length, a beam having a wider divergence angle has to be stopped down. This requires a large-diameter, rather than small-diameter, beam to be introduced into the lens. Therefore, a further improvement in the resolving power of laser beam printers is difficult to achieve unless the problem described in the preceding paragraph is solved.
The imaging lens used in the conventional optical scanner has the following problems.
1) The optical magnification of the imaging lens in the sub-scanning direction differs between the center of the lens and either edge, producing a nonuniform beam spot size in the sub-scanning direction. Additionally, an increased number of lens elements have to be used in order to provide a uniform optical magnification in the sub-scanning direction.
2) The thickness of the imaging lens in the axial direction is comparatively greater than the lens height in the sub-scanning direction, so that internal strain tends to occur during molding the lens with plastics, causing a displacement in the focal point or deterioration in the imaging characteristics.
3) The main-scanning cross section of the imaging lens is thick in the center of the lens but thin at either edge, and the difference is so great that when molding the lens with plastics, the molten resin flows unevenly to develop internal strain.
4) Since a collimated beam is admitted into the imaging lens, the latter must have a great positive refractive power, but then the thickness of the main-scanning cross section of the lens is so much greater in the center than at either edge that the thickness profile of the lens is extremely uneven.
5) Being solely composed of axially symmetric surfaces, the imaging lens has only a small degree of freedom with regard to correct aberraitions and is incapable of satisfactory correction for field curvature and scanning at uniform velocity in both the main and sub-scanning directions. In addition, the imaging lens must be composed of an increased number of lens elements in order to achieve satisfactory correction of aberrations.
6) If the reflecting face of the deflection means tilts, the scanning line will be displaced.
7) Since the imaging lens has a constant curvature in the sub-scanning direction, the curvature of the field in the sub-scanning direction cannot be adequately reduced. In addition, the imaging lens must be composed of an increased number of lens elements in order to ensure that the field curvature developing in the sub-scanning direction is adequately reduced.
8) An imaging lens formed of curved surfaces on both sides requires high production cost and, additionally, a high degree of precision is required in aligning the optical axes of both surfaces.
9) If the sub-scanning cross section of one surface of an imaging lens is linear, the degree of freedom in optical design in the sub-scanning direction must be dedicated to the correction of field curvature and it is no longer possible to produce a uniform beam spot size.
An object, therefore, of the present invention is to provide an optical scanner using an imaging lens with aspheric surfaces, the parameters of which satisfy a specified relationship in order to ensure satisfactory imaging characteristics without deformation in the shape of a beam spot while making the scanner suitable for operation at higher resolution.